i'll call it the economy tax. i'm not going to raise taxes on anyone. my plan is to bring down rates to get more people working. my priority is putting people back to work in america. i'm mitt romney and i approve this message. >>> thousands will walk towards a cure this weekend. the annual susan g. komen three- day, 06-mile walk for breast cancer is taking place. the-- 60-mile walk for breast cancer is taking place. the walk begins at the armory mall and end at the washington monument. monument. >>> we now know what caused death after giant panda cub at the national zoo. zoo officials say a liver problem is the reason the six- year-old cub did not survive. karen gray houston has a closer look. >> reporter: mei xiang was romping in her outdoor playpen, resting and sunning herself as zoo officials gathered to discuss the news. >> the cells in the liver died and the pattern would suggest lack of oxygen. >> reporter: not enough oxygen getting to the liver because the lung were not fully formed. the cub might have been premature but can't say for sure. mom